Substituted thiourea complexing agent and a method for extracting a noble metal from a matrix using the complexing agent

ABSTRACT

A substituted thiourea having the general formula 
                 
 
characterized in that each of R 1  and R 2  independently comprises an alkyl, alkaryl or aryl group or a substituted derivative thereof, and contains at least one fluorine atom, and in that each of R 3  and R 4  is selected from the group which consists of H, alkyl, alkaryl and aryl and substituted derivatives thereof, including fluorine-containing derivatives. A method for producing the substituted thiourea is disclosed, and a method for extracting a noble metal such as gold from a matrix by treating the matrix with the substituted thiourea is also disclosed.

BACKGROUND OF THE INVENTION

This invention relates to a complexing agent and in particular to newfluorinated thiourea complexing agents and fluorinated thioureacomplexing agents for use in the extraction of noble metals such asgold, platinum, silver, palladium and rhodium.

Complexing agents are widely employed in the extraction and recovery ofmetals such as gold, platinum, silver, palladium and rhodium.

For example, gold is a soft yellow metal having a melting point of 1063°C. with the highest ductility and malleability of any element. It ischemically unreactive and is not attacked by oxygen or sulphur butreacts readily with halogens or with solutions containing or generatingchlorine such as “aqua regia”. Its most common compounds exist in the(I) and (III) oxidation states.

Heretofore, the extraction of gold from one and from other solid phasessuch as in solid phase extraction has been commonly carried out by usingcyanide or thiourea as reagents. In the most commercially importantmethod for gold extraction finely crushed ore is treated with sodiumcyanide in the presence of oxygen to give a sodium gold cyanide complex,which is typically absorbed onto activated carbon. The sodium goldcyanide complex can be re-extracted later and reduced to the metal, (H.Schmidbaur, Interdisciplinary Science Reviews, 17 (3), 213, 1992 and A.Sigel and H. Sigel in “Handbook on Metals in Clinical and AnalyticalChemistry”, Ed. H. G. Seller, 1994 p388) viz:4Au+8CN⁻+O₂+2H₂O→_(^)4[Au(CN)₂]⁻+4OH³¹

However, treatment with sodium cyanide is environmentally unfriendlywhile the efficiency of the reaction can be poor and variable accordingto the ore type. Accordingly, other methods of gold and silverextraction have been developed e.g. thiourea-based extraction.Thiourea-based extractions enjoy the advantages of higher leachingefficiency, rapid leaching, adaptation to a variety of refractory oresand reduced toxicity to the environment. Accordingly, thioureation is anattractive procedure for the extraction of both gold and silver.

For example, it has been demonstrated (C. K. Chen, T. N. Lung and C. C.Lung and C. C. Wan, Hydrometallurgy, 5, 207, 1980) that employing Fe³⁺as oxidant in acid solutions resulted in leaching with thiourea whichwas ten times faster than leaching with sodium cyanide, viz:

However, excessive consumption of thiourea in the process has limitedits industrial application.

Various attempts have been made to reduce thiourea consumption. Forexample, in order to reduce thiourea consumption in gold extraction ithas been suggested (C. C. Kenna, Gold Bull, 24(4), 126, 1991) that thecomplexing of ferric ions could be utilised in reducing their oxidativepower to a level where oxidation of gold still proceeded at anacceptable rate while oxidation (and consumption) of thiourea wasgreatly reduced.

U.S. Pat. No. 5,126,038 also discloses that alkyl hydroxamic acids ortheir salts may be used to improve extraction of precious metals,including gold, from sulphide ores in combination with standard sulphideore collectors such as xanthates, substituted thioureas and the like.

G. Zuo and M. Muhammed, Separation Science and Technology, 25(13-15),1785, 1990 also describe the synthesis and characterisation of a familyof thiourea based reagents for the extraction of Au(III) and Ag(I) ionsthrough complex formation from HCl solutions and also disclose thesynthesis of several co-ordinating polymers by grafting thioureafunctional groups onto commercial macroporous polystyrene polymermatrices.

In order to avoid the use of thioureas, azacrowns have also been used tofacilitate transport of NaAu(CN)₂ into an organic phase from an aqueousphase (M. Tromp, M. Burgard, M. J. F. Leroy and M. Prevost, J. ofMembrane Science, 38, 295, 1988). In addition, Izatt et al., (R. L.Bruening, B. J. Tarbet, T. E. Krakowiak, M. L. Bruening, R. M. Izaat andJ. S. Bradshaw, Anal. Chem., 83(10), 1014, 1991 and R. L. Bruening, B.J. Tarbet, K. E. Krakowiak, R. M. Izatt and J. S. Bradshaw, J.Heterocyclic Chem., 27 347, 1990) have developed silica gel boundthia—macrocycles which have shown high selectivity for Au(III).

Supercritical fluid extraction (SFE) has developed into an attractivealternative to conventional solvent extraction to recover organiccompounds from solids in particular. A useful fluid for SFE work isliquid carbon dioxide due to its moderate critical constants(T_(c)=31.1° C., P_(c)=72.8 atm), inertness, ease of availability, lowcost and ease of final removal. However, direct extraction of metal ionsby supercritical CO₂ is very inefficient due to the changeneutralisation required and weak solute-solvent interactions.

Supercritical fluid extraction of gold has been described by S. Wang, S.Eishoni and C. M. Wal, Anal. Chem., 67, 919 1995 where Au(III) ions wereextracted by bis-triazalocrowns from wet solid matrices usingsupercritical CO₂ modified with methanol. Neutral gold complexes wereformed due to the presence of triazalo protons:

which were soluble in modified SF—CO₂. The presence of the triazoloprotons was necessary for the extraction of the metal ions to give aneutral metal ion-ligand complex:

and no extraction was possible without methanol modifier or water in thesolid phase. Supercritical CO₂ has also been utilised (E. O. Out,Separation Science and Technology 32, 6, 1107, 1997) to elute gold inthe form of NaAu(CN)₂ previously adsorbed on activated charcoalemploying tributylphosphate to facilitate charge neutralisation.However, the presence of water in the solid phase was required for theextraction while are indicated previously the use of cyanide isundesirable for environmental and safety reasons.

SUMMARY OF THE INVENTION

An object of the invention is to overcome the problems of the prior art.

A further object of the invention is to provide a complexing agent orligand for noble metal (including gold) extraction.

A further object of the invention is to provide a complexing agent orligand for noble metal (including gold) extraction.

A still further object of the invention is to provide a method forextracting noble metals (including gold) which overcomes the problems ofthe prior art.

According to the invention there is provided a thiourea having thegeneral formula:

wherein each of R¹ and R² independently comprises an alkyl, alkaryl oraryl group or a substituted derivative thereof, and contains at leastone fluorine atom, and wherein each of R³ and R⁴ is selected from thegroup which consists of H, alkyl, alkaryl and aryl, and substitutedderivatives thereof, including fluorine-containing derivatives. In oneaspect of the invention R³ and R⁴ are selected from the group consistingof alkyl, alkaryl and aryl, and substituted derivatives thereofincluding fluorine-containing derivatives. The fluorinated derivativesof the invention are extremely useful for analytical assays for thedetermination of gold levels, and for gold recovery, as well as fordetermination of platinum, silver, palladium and rhodium levels and fortheir recovery.

Preferably, R¹ comprises

R³ comprises H and R⁴ comprises H.

Alternatively, R¹ comprises

R³ comprises H and R⁴ comprises H.

The invention also extends to a method of producing a fluorinatedthiourea comprising reacting a compound of general formula

with a compound of general formula

where both R¹ and R² contain fluorine.

The invention also relates to a method for extracting gold from a matrixcomprising treating the matrix with a thiourea having the generalformula

where R¹ and R² comprise a fluorine containing alkyl, alkaryl, aryl orsubstituted derivatives thereof and R³ and R⁴ are selected from thegroup comprising H, alkyl, aryl or substituted derivatives thereof andsubjecting the matrix to supercritical fluid extraction (SFE).

Preferably, the supercritical fluid used in the extraction comprisesliquid carbon dioxide.

Preferably, R¹ comprises

R³ comprises H and R⁴ comprises H where a ≧1 and b=0−6.

In one embodiment of the invention the gold is extracted in the presenceof an oxidant. Suitably, the oxidant comprises Fe (III) ions.

Advantageously, b>3, i.e. b=4, 5 or 6.

The invention also extends to the use of a fluorinated thiourea of thegeneral formula

where R¹ and R² comprise a fluorine containing alkyl, alkaryl, aryl orsubstituted derivatives thereof and R³ and R⁴ are selected from thegroup comprising H, alkyl, aryl, alkaryl or substituted derivativesthereof in the extraction of a noble metal, including gold, platinum,silver, palladium and rhodium from a matrix.

Surprisingly, it has been found that fluorinated thioureas of thegeneral formula:

wherein a≧1 and b=0−6 efficiently extract Au(I) and Au(III) from a solidmatrix in unmodified supercritical CO₂ and furthermore may extract goldfrom a solid containing gold in its elemental form in the presence of anoxidant. Fe(III) ions are particularly suitable as oxidants. Theaddition of modifiers or protons is not required and extraction can becarried out using fluorinated thioureas alone.

In contradistinction, thiourea itself does not form a neutral complexwith gold. It forms Au [SC(NH₂)₂]₂ ⁺.

In a preferred embodiment of the invention b>3, i.e. b=4, 5 or 6.

The fluorinated thioureas of the invention have a high solubility insupercritical CO₂ and are extremely efficient at solubilising andcarrying noble metals such as gold for the purposes of extraction,recovery, deposition or impregnation.

The fluorinated thioureas can be synthesised in a simple one-stepprocess by the reaction of:

with the appropriate amine NH₂CH₂(CF₂)_(b)CF₃ where b=0 or 2 to 6 in aroom temperature (exothermic) reaction and recrystallisation frompetroleum ether (100-120) to give colourless products in 62 to 81%yields.

The compositions of the invention can therefore be formed by the simplereaction of:

where both R¹ and R² contain fluorine.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the invention will now be described by way ofExample only, having regard to the following data and examples.

COMPARATIVE EXAMPLES

The aforementioned fluorinated thiourea is known from the prior art andis commercially available from FLUOROCHEM.

In a round bottom flask 6.46 g (0.05 mole) of octyl-1-amine was added to3.65 g (0.05 mole) melted methyl isothiocyanate, with stirring, undernitrogen in an ice bath. A rapid exothermic reaction ensued and thereaction was allowed to reach room temperature overnight. The colourlesssolid product (10.02 g, 99% yield) was recrystallised from 100-120° C.petroleum ether to give 9.99 g of N-methyl,N′-octyl thiourea as a whitesolid. Note: The oil, which settles out on cooling, solidifies onstanding, yield 99%. Elemental Analysis for C₁₀H₂₂N₂S Calculated: C:59.36, H; 10.96, N; 13.84, S; 15.84% Found: C: 59.60, H 10.99, N; 13.50,S; 16.11%

In a round bottom flask 9.27 g (0.05 mole) dodecyl-1-amine was added to3.65 g (0.05 mole) of melted methyl-isothiocyanate with stirring undernitrogen. A rapid exothermic reaction ensued and the reaction mixturewas allowed to reach room temperature overnight. The colorless solidproduct 12.45 g (96% yield) was recrystallised from 100-120° C.petroleum ether to give 12.27 g (95% yield) of N-methyl, N′-dodecylthiourea as white crystals. Elemental Analysis for C₁₄H₃₀N₂S Calculated:C: 65.05, H; 11.70, N; 10.84, S; 12.40% Found: C: 64.90, H; 11.67, N;11.10, S; 12.78%

Example 1

To 0.518 g (0.0026 mole) 1H, 1H-heptafluorobutylamine (Fluorochem (TradeMark) Product FO4396) in a round bottom flask was added 0.705 g (0.0026mole) 3,5-di(trifluoromethyl) phenylisothiocyanate (Fluorochem (TradeMark) Product F03115B). After stirring for one minute the miscibleliquids solidified to a colourless solid in an exothemic reaction. Afterthe reaction mixture had cooled it was allowed to stand for 1 hour atroom temperature and the product was recrystallised from petroleum ether(100-120) to give 0.972 g of pure product (80% yield) as colourlesscrystals, mp 130-132° C.

Elemental Analysis for C₁₃H₇N₂SF₁₃: Calculated: C: 33.20, H; 1.50, N;5.96, Found C: 33.05, H; 1.49, N; 8.12%

Example 2

To 0.705 g (0.0026 mole) 3,5-di(trifluoromethyl)-phenylisothiocyanate ina round bottom flask cooled in an ice bath, with stirring undernitrogen, was added 0.281 g (0.00283 mole) trifluoroethyl amine (Aldrich(Trade Mark) produce 26,904-2). After a short period an exothermicreaction occurred to give a colourless solid. The reaction mixture wasthen allowed to warm to room temperature and was left for 1 hour under astream of dry nitrogen to remove excess volatile amine (bp 36° C.). Thesolid was recrystallised from petroleum ether (100-120) to give a pureproduct as fluffy colourless crystals 0.80 g (62% yield), m.p. 133-136°C. Elemental Analysis for C₁₁H₇N₂SF₂: Calculated: C: 35.64, H; 2.04, N;7.55, Found C: 35.94, H; 2.20, N; 7.73%

Example 3(a)

To 3.28 (0.0121 mole) 3,5-di(trifluoromethyl)-phenylisothiocyanate in around bottom flask was added 4.82 g (0.0121 mole) 1H,1H-perfluoro-octylamine Lancaster (Trade Mark) product 16845 withstirring. An exothermic reaction rapidly ensued and after cooling toroom temperature was allowed to remain for 2 hours. The white solidproduct was recrystallised from 100-120° C. petroleum ether to give 6.50g pure product (81% yield) as a colorless crystalline solid. ElementalAnalysis: C₁₇H₇N₂SF₂: Calculated: C: 30.46, H; 1.05, N; 4.18; Found:C:30.60, H; 1.16, N; 4.40%

Example 3(b)

In a round bottomed flask 5.92 g (0.03 mole) of 1H, 1H-heptafluorobutylamine was added to 2.18 g (0.30 mole) of melted methyl-isothiocyanatewith stirring under nitrogen. An exothermic reaction ensued and thereaction mixture was allowed to reach room temperature overnight. Thecolourless product 8.10 g (100% yield) was recrystallised from 100-120°C. petroleum ether to give N-methyl, N′-heptafluorobutyl thiourea, 6.91g, as a white solid in 85% yield. Elemental Analysis for C₈H₇N₂SF₇Calculated: C: 26.48, H; 2.59, N; 10.29, S; 11.78% Found: C: 26.71, H;2.57, N; 10.50, S; 12.25%

Example 4

The solubility of the ligand of the comparative Examples (a)-(c) insupercritical CO₂ was compared with the solubilities of the ligands ofExamples 1 and 2 and 3 in supercritical CO₂.

In each case, a weighed amount of the ligand of the respectivecomparative Example of approximately 60 mg was placed in a glass tube (2cm×0.5 cm i.d.) and plugged with glass wool at both ends. The glass tubewas placed inside the extraction vessel and statically extracted for 30minutes. The inlet valve for SF—CO₂ was then closed and the outlet valveopened into a collecting solution. The loss of weight of the glass tubeafter SFE corresponded to the solubility of the ligand in 2.2 ml 100%SF—CO₂. The procedure was carried out at 60° C. and two differentpressures namely 200 and 300 atmospheres.

In all three cases, most of the ligand appeared to remain in the glasstube indicating poor solubility in SF—CO₂.

Solubility of the Ligand of Example 1 in Supercritical CO₂:

The procedure outlined above in Example 4 was repeated for the ligand ofExample 1. This time none of the ligand remained in the glass tube,indicating excellent solubility in SF—CO₂ at both pressures.

Solubility of the Ligand of Example 2 in Supercritical CO₂:

The procedure outlined above in Example 4 was repeated for the ligand ofExample 2. Again, none of the ligand remained in the glass tube,indicating excellent solubility in SF—CO₂ at both pressures.

Solubility of the Ligand of Example 3(a) in Supercritical CO₂:

The procedure outlined above in Example 4 was repeated for the ligand ofExample 3(a). Again, none of the ligand remained in the glass tube,indicating excellent solubility in SF—CO₂ at both pressures.

Solubility of the Ligand of Example 3(b) in Supercritical CO₂:

The procedure outlined above in Example 4 was repeated for the ligand ofExample 3(b). Again, none of the ligand remained in the glass tube,indicating excellent solubility in SF—CO₂ at both pressures. Largeramounts of ligand 3(b) were used and solubilities in excess of 0.7 Mwere thus found at both pressures. Thus the newly synthesised thioureaswere found to be highly soluble in supercritical CO₂ compared to thefluorinated and two non-fluorinated thioureas of the ComparativeExamples (a)-(c).

A number of experiments were carried out to demonstrate the extractionefficiencies of the fluorinated thioureas of the invention.

Example 5(a) (Comparative) Supercritical Fluid Extraction of Au(III) asAuCl₄ ⁻ using the Compound of the Comparative Example (a)

Gold Au(III) extraction by the ligand of the comparative Example (a) wasinvestigated employing a BDH Gold (III) standard containing 1000 ppmAu(III) (aqueous AuCl₄ ⁻). Thus 60 μl of solution Au(III) containing3.05×10⁻⁷ moles Au(III) was applied to a 3 cm diameter filter paper. Thefilter paper was allowed to dry and then placed in a glass tube (2cm×0.5 cm i.d.), plugged with glass wool at both ends. 20 mg of ligandof the comparative Example (a) (in excess of over 200 fold over Au(III)level) was then placed in the same glass tube and plugged with glasswool. The temperature of the extraction vessel was then set at 60° C.and the pressure was varied as indicated in Table 1.

The extraction vessel was statically extracted for 20 minutes and thendynamically extracted into a collecting solvent of 4 ml methanol for 15minutes (0.8 ml CO₂/minute flow rate). The methanol solution was thenmade up to 10 ml using additional methanol. Levels of gold in solutionwere then determined by atomic absorption spectroscopy. The procedurewas carried out at different pressures from 200-400 atm. The followingextraction percentages were obtained:

TABLE 1 Pressure SF—CO₂ Atomic Absorption Extraction (atm) (A.U.)* (%)200 0.000 0 250 0.000 0 300 0.001 ˜0.0 350 0.000 0 400 0.024 6.7 *The %extraction is calculated with reference to the Atomic Absorption readingobtained for 10 ml of collecting solution spiked directly with 60 μl ofthe 1000 ppm Au(III) standard. (For example for the data presented inTable 1 a standard of 60 μg/10 ml = 6 ppm Au, gave an absorption valueof 0.359, representing 100% extraction. Note: such recordings ofstandard values were carried out alongside the sample analysed on thesame day.)

400 atm was the only pressure of SF—CO₂ to give detectable Au(III)extraction. In all runs most of the ligand appeared to remain in theglass tube indicating poor solubility in SF—CO₂.

Example 5(b) (Comparative) Supercritical Fluid Extraction of Au(III) asAuCl₄ ⁻ using the Compound of the Comparative Example (b)

The procedure of Example 5(a) was repeated for Comparative Example (b)to give 3.2% gold extraction at 250 atmospheres and 2.0% at 450atmospheres pressure. Most of the ligand appeared to remain in the glasstube after all runs indicating poor solubility in SF—CO₂.

Example 5(c) (Comparative) Supercritical Fluid Extraction of Au(III) asAuCl₄ ⁻ using the Compound of the Comparative Example (c)

The procedure of Example 5(a) was repeated for Comparative Example (c)to give 2.6% gold extraction at 250 atmospheres and 1.3% at 450atmospheres pressure. Most of the ligand appeared to remain in the glasstube after all runs indicating poor solubility in SF—CO₂.

Example 6 (Comparative)

The procedure in Example 5(a) was repeated except the temperature of theextraction procedure was varied from 60-120° C., while maintaining thepressure of the extractor at 400 atm. The following results wereobtained:

TABLE 2 Temperature Atomic Absorption Extraction (° C.) (A.U.)* (%) 600.027 8.8 80 0.002 0.7 100 0.006 2.0 120 0.005 1.6 *A standard of 6 ppmAu, gave an absorption value of 0.305 = 100%

The % extraction of gold remained low.

Example 7

The procedure in Example 6 was repeated with the ligand from Example 1being used in place of the ligand of the Comparative Example (a). Thefollowing % extraction values were obtained at differing pressures forextraction of Au(III):

TABLE 3 Pressure SF—CO₂ Extraction (atm) (%) 200 61.2 250 92.7 300 83.7350 75.3 400 78.1

Percentage extraction with the ligand of the invention was thereforeexcellent at 92.7% compared with the poor extraction (<10%) with theligands of the comparative Examples.

Example 8

Reduction of Au(III) to Au(O) was accomplished by treatment withhydroxylamine hydrochloride followed by sodium hydroxide.

The procedure in Example 7 was repeated except the conditions werealtered to 60° C. at 250 atm. The 60 μl of AuCl₄ ⁻ was replaced by 60 μlof a well mixed even suspension of Au(O) applied to the filter paperwhich was allowed to dry to give a black-blue colour.

The collected methanol solutions, made up to 10 ml as before, wereanalysed by Atomic Absorption Spectroscopy as before, giving thefollowing % extraction values:

TABLE 4 Pressure SF—CO₂ Atomic Absorption Extraction (atm) (A.U)* (%)200 0.001 ˜0.0 250 0.002 ˜0.0 300 0.0001 ˜0.0 350 0.002 ˜0.0 400 0.002˜0.0

Accordingly, the ligand of Example 1 did not extract Au(O). Moreover, atthe end of the runs the blue-black colour of Au(O) remained on thefilter paper and no ligand remained in the tube. However, as describedfurther below the gold could be extracted following oxidation of Au(O)to either Au(I) or Au(II).

Example 9

The procedure in Example 8 was repeated but 40 mg of solid Au(O) wasused in place of the Au(O) suspension deposited on the filter paper togive an identical result to Example 8.

Example 10

The procedure of Example 9 was repeated utilising 20 mg of the ligandExample 1 except 11.2 mg of solid Au(O) was used and on two occasionsthe Au(O) was first oxidised to Au(I) by spiking 60 μl of Fe(III) (1000ppm stock solution) onto the solid Au(O) directly and then allowed todry. In the case where Fe(III) had been added when the collectedmethanol solution (as usual made up to 10 ml) was analysed by atomicabsorption a large signal was obtained (0.252) and (0.253) indicatingextraction of Au(I). Fe(III) was the limiting reagent as:Fe(III)+Au(O)→Fe(II)+Au(I)and therefore 60 μl 1000 ppm Fe(III) (1.075 μmole) is equivalent to 212μl of 1000 ppm Au(I) (0.2 mg Au).

Table 5 below outlines the results obtained following application ofFe(III):

TABLE 5 Atomic Absorption Extraction Sample (A.U.)* (%) 11.2 mg Au(O) +0.001 0 20 mg ligand of Example 1 11.2 mg Au(O) + 0.252 20.0 60 μlFe(III) + 20 mg ligand of Example 1 11.2 mg Au(O) + 0.253 20.0 60 μlFe(III) + 20 mg ligand of Example 1

A 212 μl aliquot of 1000 pp, Au(III) standard solution gave anabsorption value of 1.263=100%.

Accordingly, Au(O) has been successfully oxidised by Fe(III) to giveAu(I) which has been extracted with the fluorinated ligand of theinvention. That only 20% of the theoretically freed gold (by Fe) wasfinally detected by atomic absorption is not surprising in view of thefact that the Fe(III) was applied as a 1000 ppm aqueous nitrate solutionto the solid Au(O) on the filter paper without thorough mixing.Nevertheless, the method can be employed successfully as a qualitativetest for Au(O).

Example 11

The procedure of Example 7 was repeated employing the ligand of Example2 in place of the ligand of Example 1 and utilising 50 μl Au(III)standard solution in place of 60 μl to give the following % extractionresults at differing pressures of SF—CO₂ at 60° C.

TABLE 6 Extraction Atmosphere SF—CO₂ (%) 200 41 250 51 300 22 400 14

Example 12

The procedure of Example 11 was repeated except Au(III) standard wasreplaced by 5.7 mg solid Au(O) and extraction was determined underpreviously optimised conditions 60° C./250 atm SF—CO₂. Table 7summarises the results:

TABLE 7 Extraction Sample (%) Au(O) + ligand ˜3

Example 13

Above Example 12 was repeated except that 30 μl 1000 ppm Fe(III)standard (aqueous nitrate) was spiked onto the Au(O) prior to extractionwith SF—CO₂ (250 atm/60° C.). Table 8 below summarises the resultsobtained.

TABLE 8 Extraction Sample (%) Au(O) + 30 μl Fe(III) + 57 20 mg ligand

The above percentage is based on the Fe(III) oxidisable quality of goldi.e. Fe(III) is the limiting reagent.

57% of the “freed” gold Au(I) was therefore extracted with thefluorinated ligand of the invention.

Advantages of the invention include (but are not limited to) thefollowing:

The linear fluorinated thioureas of the invention therefore have theunexpected property of extracting Au(III) in supercritical CO₂. Inaddition Au(I) may be extracted from Au(O) (in its elemental state) byprior treatment with Fe(III).

The complexing agents and extraction methods of the invention are highlyefficient and do not require the use of cyanides. In addition, thefluorinated thioureas of the invention facilitate the extraction ofnoble metals (including gold, platinum, silver, palladium and rhodium)without excessive thiourea consumption.

Moreover, extraction of noble metals (including gold, platinum, silver,palladium and rhodium) using fluorinated thioureas and supercriticalfluid can be effected without requiring the addition of modifiers,protons and the like.

The invention is not limited to the embodiments herein described whichmay be varied in construction and detail.

1. A substituted thiourea having the general formula

wherein R¹ comprises an alkyl, alkaryl or aryl group or a substitutedderivative thereof, and contains at least one fluorine atom, R² is—CH₂—CF₂—CF₂—CF₃, and each of R³ and R⁴ is selected from the groupconsisting of H, alkyl, alkrayl, aryl, substituted derivatives of H,alkyl, alkaryl or aryl, and fluorine-containing derivatives of H, alkyl,alkaryl or aryl.
 2. A thiourea according to claim 1 wherein R¹ is


3. A thiourea according to claim 1 wherein R³ is H.
 4. A thioureaaccording to claim 1 wherein R⁴ is H.
 5. A thiourea according to claim 2wherein the substituted thiourea is of the formula


6. A method for extracting a noble metal from a matrix, the methodcomprising the steps of treating the matrix with a substituted thioureahaving the general formula

wherein R¹ and R² independently comprise an alkyl, alkaryl or aryl groupor a substituted derivative thereof, and contain at least one fluorineatom, and each of R³ and R⁴ is selected from the group consisting of H,alkyl, alkaryl, aryl, substituted derivatives of H, alkyl, alkaryl oraryl, and fluorine-containing derivatives of H, alkyl, alkaryl or aryl,and subjecting the thus treated matrix to supercritical fluidextraction.
 7. A method according to claim 6 wherein the noble metal isgold, platinum, silver, palladium or rhodium.
 8. A method according toclaim 6 wherein the supercritical fluid is supercritical carbon dioxide.9. A method according to claim 6 wherein the treatment with substitutedthiourea is performed in the presence of an oxidant.
 10. A methodaccording to claim 9 wherein the oxidant comprises ferric (Fe^(III))ions.
 11. A method according to claim 6 wherein the treatment andextraction are carried out at room temperature and are followed byrecrystallisation of the product from petroleum ether at a temperaturein the range of 100° C. to 120° C.
 12. A substituted thiourea having thegeneral formula


13. A method of extracting gold, platinum, silver, palladium or rhodiumfrom a matrix comprising: treating the matrix with a substitutedthiourea, and subjecting the treated matrix to supercritical fluidextraction, wherein the substituted thiourea has the general formula

wherein R¹ and R² independently comprise an alkyl, alkaryl or aryl groupor a substituted derivative thereof, and contain at least one fluorineatom, and each of R³ and R⁴ is selected from the group consisting of H,alkyl, alkaryl, aryl, substituted derivatives of H, alkyl, alkaryl oraryl, and fluorine-containing derivatives of H, alkyl, alkaryl or aryl.14. A method of solubilising and carrying noble metals for deposition orimpregnation thereof, comprising: treating a matrix containing noblemetals with a substituted thiourea, and subjecting the treated matrix tosupercritical fluid extraction, wherein the substituted thiourea has thegeneral formula

wherein R¹ and R² independently comprise an alkyl, alkaryl or aryl groupor a substituted derivative thereof, and contain at least one fluorineatom, and each of R³ and R⁴ is selected from the group consisting of H,alkyl, alkaryl, aryl, substituted derivatives of H, alkyl, alkaryl oraryl, and fluorine-containing derivatives of H, alkyl, alkaryl or aryl.